The forthcoming Intergovernmental Panel on Climate Change (IPCC) Summary for Policymakers, it has been reported, states that the rate of global warming has slowed over the last 15 years. It also argues that estimates of eventual warming from a doubling of carbon dioxide levels in the atmosphere are lower than was earlier thought. Taken individually, each of these assertions is a partial narration of ongoing climate processes. Read together, they carry the danger of fostering complacency, both about the current rate of global warming and the urgency in avoiding dangerous levels of warming.
There have been at least three theories in recent climate science literature seeking to explain the slowdown, or “hiatus,” in global warming. Global warming is measured by taking an average of near-surface air temperatures all over the globe throughout the year, but this does not account for the heat trapped by greenhouse gases that is transported into the deeper oceans. Warming of the ocean waters below 700 metres has been exceptional in recent years. A study in Geophysical Research Letters says that “depths below 700 metres have become much more strongly involved in the heat uptake after 1998, and subsequently account for 30% of the ocean warming,” precisely the period in which surface warming has slowed down. But despite being transported into the deeper oceans, much of this heat energy will show up as warming sooner or later.
Another proposition is that a prolonged La Niña-like cooling in the tropical Pacific has lessened the impact of greenhouse gases by 0.15° Celsius globally in the recent decade. It is a natural variability and, if this is the cause, the slowdown will be temporary, as a recent paper argues (Yu Kosaka and Shang-Ping Xie, ‘Recent Global Warming Hiatus Tied to Equatorial Pacific Surface Cooling’, Nature , doi: 10.1038/nature12534). A third theory is that near-surface warming is being masked by an increased generation of aerosols, caused by greater manufacturing occurring in China in this period and, to a lesser degree, India. This particulate pollution is harmful to human health but has a cooling effect in climate terms. In the decades after World War II as well, aerosols from dirty manufacturing processes — then in the developed world — slowed surface warming despite one of the most rapid rates in carbon dioxide emissions growth. Unlike CO though, aerosols have a lifespan of a few days; clean up your industrial act, and their cooling effect promptly disappears.
These varied explanations help form a more complete picture of ongoing climate processes. One assumes that this more complex picture would be presented, if not in the AR5 Summary for Policymakers, then in the Technical Summary, which in IPCC’s AR4 2007 was over four times as long as the former. It would be premature to rush to a definite opinion before seeing what these documents say, and hearing independent scientific opinion on them. The half has not been told us.
The second major revelation is that the lower end of eventual warming from a doubling of carbon dioxide levels has been reduced from 2°C in the IPCC’s 2007 AR4 report, to 1.5°C. Some have argued, though, that most such estimates do not include slow feedbacks. Feedbacks, in the climate context, are ecosystem responses to global warming that usually cause further warming. The effects of slow feedbacks — such as reduced albedo reflectivity from the great ice sheets in Antarctica and Greenland, and greenhouse gas releases from the oceans on their getting warmer (in contrast to absorbing them now) — take much longer to play out. Including slow feedbacks in their analysis led James Hansen, among the world’s most respected climate scientists, and others to conclude that “global climate sensitivity including the slow surface albedo feedback is … 6 degrees Celsius for doubled CO”. Carbon dioxide may be much more potent than we realise. Again, perhaps it is best to keep conclusions in abeyance until we fully hear what the IPCC has to say.
Melting in the Arctic
What is really worrying is the complacency that these two points — that warming is slowing and CO is less potent — read together may engender. As it is, sections of the Indian political class are not exactly known for their alacrity in responding to crises faced by the poor. Making them respond with greater urgency becomes all the more difficult if complacency about global warming spreads among political organisations and members of the public at large. I have already been asked, at two talks in the last one week: but the warming is slowing down, isn’t it? Even as we speak, the world is facing the dead-certain prospect of largely ice-free Arctic summers for the first time in at least 2.5 million years. Ice-free Arctic summers will nudge us even closer to dangerous or even irreversible warming. The need for that urgency has never been greater.
(Nagraj Adve works and writes on issues related to global warming. E-mail: firstname.lastname@example.org )